586 research outputs found
Multicore structures and the splitting and merging of eddies in global oceans from satellite altimeter data
This study investigated the statistics of eddy splitting and
merging in the global oceans based on 23Â years of altimetry data. Multicore
structures were identified using an improved geometric closed-contour
algorithm of sea surface height. Splitting and merging events were discerned
from continuous time series maps of sea level anomalies. Multicore
structures represent an intermediate stage in the process of eddy evolution,
similar to the generation of multiple nuclei in a cell as a preparatory
phase for cell division. Generally, splitting or merging events can
substantially change (by a factor of 2 or more) the eddy scale, amplitude, and
eddy kinetic energy. Specifically, merging (splitting) generally causes an
increase (decrease) of eddy properties. Multicore eddies were found to tend
to split into two eddies with different intensities. Similarly, eddy merging
is not an interaction of two equal-intensity eddies, and it tends to
manifest as a strong eddy merging with a weaker one. A hybrid tracking
strategy based on the eddy overlap ratio, considering both multicore and
single-core eddies, was used to confirm splitting and merging events
globally. The census revealed that eddy splitting and merging do not always
occur most frequently in eddy-rich regions; e.g., their frequencies of
occurrence in the Antarctic Circumpolar Current and western boundary
currents were found to be greater than in midlatitude regions
(20–35∘) to the north and south. Eddy splitting and merging
are caused primarily by an unstable configuration of multicore structures
due to obvious current– or eddy–topography interaction, strong current
variation, and eddy–mean flow interaction.</p
Experimental time-domain controlled source electromagnetic induction for highly conductive targets detection and discrimination
The response of geological materials at the scale of meters and the response
of buried targets of different shapes and sizes using controlled-source electromagnetic
induction (CSEM) is investigated. This dissertation focuses on three topics; i) frac-
tal properties on electric conductivity data from near-surface geology and processing
techniques for enhancing man-made target responses, ii) non-linear inversion of spa-
tiotemporal data using continuation method, and iii) classification of CSEM transient
and spatiotemporal data.
In the first topic, apparent conductivity profiles and maps were studied to de-
termine self-affine properties of the geological noise and the effects of man-made con-
ductive metal targets. 2-D Fourier transform and omnidirectional variograms showed
that variations in apparent conductivity exhibit self-affinity, corresponding to frac-
tional Brownian motion. Self-affinity no longer holds when targets are buried in the
near-surface, making feasible the use of spectral methods to determine their pres-
ence. The difference between the geology and target responses can be exploited using
wavelet decomposition. A series of experiments showed that wavelet filtering is able
to separate target responses from the geological background.
In the second topic, a continuation-based inversion method approach is adopted,
based on path-tracking in model space, to solve the non-linear least squares prob-
lem for unexploded ordnance (UXO) data. The model corresponds to a stretched-
exponential decay of eddy currents induced in a magnetic spheroid. The fast inversion of actual field multi-receiver CSEM responses of inert, buried ordnance is also shown.
Software based on the continuation method could be installed within a multi-receiver
CSEM sensor and used for near-real-time UXO decision.
In the third topic, unsupervised self-organizing maps (SOM) were adapted for
data clustering and classification. The use of self-organizing maps (SOM) for central-
loop CSEM transients shows potential capability to perform classification, discrimi-
nating background and non-dangerous items (clutter) data from, for instance, unex-
ploded ordnance. Implementation of a merge SOM algorithm showed that clustering
and classification of spatiotemporal CSEM data is possible. The ability to extract tar-
get signals from a background-contaminated pattern is desired to avoid dealing with
forward models containing subsurface response or to implement processing algorithm
to remove, to some degree, the effects of background response and the target-host
interactions
A Lagrangian Analysis of Midlatitude Air-Sea Interaction Associated with Mesoscale Oceanic Eddies
Oceanic eddies, approximately 80 km in diameter in the mid-latitude Atlantic Ocean, are frequently generated along the Gulf Stream. These eddies contain a sea surface temperature anomaly as they spin and break away from the main current. Oceanic eddies have been shown to effect the lower atmosphere in several regions in prior studies, influencing rainfall rates, cloud cover, and surface wind speeds in satellite observations. Eddy features are located using an automated detection algorithm and the atmosphere surrounding each eddy's location is studied. Coupled global and regional models, utilizing both atmospheric and oceanic components, are tested to investigate their capability of demonstrating the atmospheric response compared to observations.
It is found that the atmospheric influence of oceanic eddies is primarily limited to the boundary layer, with an exception for vertical motion influence that can extend beyond it. Eddies with a positive sea surface temperature anomaly core increase the formation of low-level clouds and rainfall slightly downstream of the eddy center, with clouds forming consistently at a height of approximately 850 hPa in models and reanalysis data. The vertical mixing mechanism is shown to be the cause of the wind response to oceanic eddy influence. This is based on a dipolar pattern of wind divergence aligning with the sea surface temperature gradient, along with a change in horizontal wind speed close to the top of the boundary layer due to vertical momentum transfer.
The results show coupled models overestimate the coupling strength between sea surface temperatures and the atmospheric response compared to observations. This indicates coupled models are capable of adequately simulating the atmospheric response, as long as the oceanic component's resolution is eddy resolving. In the regional model, parameterized convection showed a very similar result to explicitly resolving the convection. Therefore, the atmospheric response to oceanic eddies can be simulated properly even if the convection has been parameterized
Earth resources: A continuing bibliography with indexes (issue 52)
This bibliography lists 454 reports, articles, and other documents introduced into the NASA scientific and technical information system between October 1 and December 31, 1986. Emphasis is placed on the use of remote sensing and geophysical instrumentation in spacecraft and aircraft to survey and inventory natural resources and urban areas. Subject matter is grouped according to agriculture and forestry, environmental changes and cultural resources, geodesy and cartography, geology and mineral resources, hydrology and water management, data processing and distribution systems, instrumentation and sensors, and economic analysis
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